Workpiece holding apparatus, workpiece holding method, program, and control apparatus

ABSTRACT

To calculate a holding position of a workpiece with high accuracy and place the workpiece in a placement position with high accuracy based on the holding position. A workpiece holding apparatus includes: holding means for holding a workpiece; first information acquisition means for acquiring three-dimensional information of the workpiece held by the holding means; position calculation means for calculating a lowest center point of the workpiece as position information of the workpiece based on the three-dimensional information of the workpiece acquired by the first information acquisition means; and control means for calculating a placement position where the workpiece is to be placed based on the position information of the workpiece calculated by the position calculation means and controlling, based on the placement position, the holding means so as to move the workpiece to the placement position and place the workpiece in the placement position.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2021-168015, filed on Oct. 13, 2021, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The present disclosure relates to a workpiece holding apparatus, aworkpiece holding method, a program, and a control apparatus for holdingand placing a workpiece.

While a hand of a workpiece holding apparatus holds a workpiece, theworkpiece holding apparatus captures an image of the held workpiece,performs image processing on the captured image of the workpiece,calculates position information of the workpiece, and places theworkpiece based on the calculated position information has been known(see, for example, Japanese Unexamined Patent Application PublicationNo. 2011-161395).

SUMMARY

In the above workpiece holding apparatus, even when a holding positionof the workpiece changes two-dimensionally, the holding position of theworkpiece can be calculated by reading an alignment mark of theworkpiece. However, for example, when the workpiece is attracted by amagnet or the like and held, the holding position of the workpiecechanges three-dimensionally. In this case, there is a possibility thatthe holding position of the workpiece cannot be calculated with highaccuracy by the above method for reading an alignment mark and thus theworkpiece cannot be placed with high accuracy in a placement positionwhere the workpiece is to be placed.

The present disclosure has been made to solve such problems, and one ofthe main objects thereof is to provide an workpiece holding apparatus, aworkpiece holding method, a program, and a control apparatus capable ofcalculating a holding position of a workpiece with high accuracy andplacing the workpiece in a placement position with high accuracy basedon the holding position.

In order to achieve the above-described object, a first exemplary aspectis a workpiece holding apparatus including:

-   holding means for holding a workpiece;-   first information acquisition means for acquiring three-dimensional    information of the workpiece held by the holding means;-   position calculation means for calculating a lowest center point of    the workpiece as position information of the workpiece based on the    three-dimensional information of the workpiece acquired by the first    information acquisition means; and-   control means for calculating a placement position where the    workpiece is to be placed based on the position information of the    workpiece calculated by the position calculation means and    controlling, based on the placement position, the holding means so    as to move the workpiece to the placement position and place the    workpiece in the placement position.

In this aspect, the workpiece holding apparatus may further includesecond information acquisition means for acquiring three-dimensionalinformation of a placement area where the workpiece is to be placed.

In this aspect, the first information acquisition means may acquireinformation about a height direction of the workpiece, and

the control means may change the placement position based on theinformation about the height direction of the workpiece acquired by thefirst information acquisition means.

In this aspect, a plurality of spiky projections having differentheights are provided in the placement area where the workpiece is to beplaced,

-   the control means may set a position between the adjacent    projections as the placement position of the workpiece, and-   the control means may change the placement position of the workpiece    in the placement area based on the information about the height    direction of the workpiece acquired by the first information    acquisition means.

In this aspect, a length of the projection in a height direction may beset in accordance with a length of the workpiece in the heightdirection, the workpiece being placed in the placement area.

In order to achieve the above-described object, another exemplary aspectmay be a workpiece holding method including:

-   acquiring three-dimensional information of a workpiece held by    holding means;-   calculating a lowest center point of the workpiece as position    information of the workpiece based on the acquired three-dimensional    information; and-   calculating a placement position where the workpiece is to be placed    based on the calculated position information of the workpiece and    controlling, based on the placement position, the holding means so    as to move the workpiece to the placement position and place the    workpiece in the placement position.

In order to achieve the above-described object, another exemplary aspectmay be a program for causing a computer to perform:

-   a process of acquiring three-dimensional information of a workpiece    held by holding means;-   a process of calculating a lowest center point of the workpiece as    position information of the workpiece based on the acquired    three-dimensional information; and-   a process of calculating a placement position where the workpiece is    to be placed based on the calculated position information of the    workpiece and controlling, based on the placement position, the    holding means so as to move the workpiece to the placement position    and place the workpiece in the placement position.

In order to achieve the above-described object, another exemplary aspectmay be a control apparatus including:

-   position calculation means for calculating a lowest center point of    a workpiece as position information of the workpiece based on    three-dimensional information of the workpiece acquired by first    information acquisition means; and-   control means for calculating a placement position where the    workpiece is to be placed based on the position information of the    workpiece calculated by the position calculation means and    controlling, based on the placement position, holding means so as to    move the workpiece to the placement position and place the workpiece    in the placement position.

According to the present disclosure, it is possible to provide aworkpiece holding apparatus, a workpiece holding method, a program, anda control apparatus capable of calculating a holding position of aworkpiece with high accuracy and placing the workpiece in a placementposition with high accuracy based on the holding position.

The above and other objects, features and advantages of the presentdisclosure will become more fully understood from the detaileddescription given hereinbelow and the accompanying drawings which aregiven by way of illustration only, and thus are not to be considered aslimiting the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a schematic system configuration of aworkpiece holding apparatus according to an embodiment;

FIG. 2 is a block diagram showing a schematic system configuration of acontrol apparatus according to the embodiment;

FIG. 3 is a diagram showing a position of the lowest center point of aworkpiece;

FIG. 4 is a diagram showing a placement position of a workpiece inaccordance with a length of the workpiece in the height direction;

FIG. 5 is a diagram showing outer regions of other workpieces that arealready placed in a placement area; and

FIG. 6 is a flowchart showing a flow of processes performed by a methodfor holding a workpiece according to the embodiment.

DESCRIPTION OF EMBODIMENTS

An embodiment according to the present disclosure will be describedhereinafter with reference to the drawings. FIG. 1 is a block diagramshowing a schematic system configuration of a workpiece holdingapparatus according to this embodiment. A workpiece holding apparatus 1according to this embodiment can attract (e.g., suck), hold, and moveeach workpiece in turn from among a plurality of workpieces placed in athree-dimensional space, and then place each workpiece in a placementarea where it is to be placed. The plurality of workpieces arecomponents or the like that are, for example, contained in a box orstacked on a flat surface in a random manner. The placement area of theworkpiece (i.e., the placement area where the workpiece is to be placed)is, for example, the inside of a box or the top of a flat plate. Notethat the plurality of workpieces includes components having arbitraryshapes, such as a planar shape and a three-dimensional shape.

The workpiece holding apparatus 1 according to this embodiment includesa robot arm 2, a control apparatus 3, a first vision sensor 4, and asecond vision sensor 5.

The robot arm 2 is a specific example of holding means. For example, therobot arm 2 is configured as an articulated arm including a plurality oflinks 21, joint parts (such as a wrist joint, an elbow joint, and ashoulder joint) 22 each of which rotatably connects a corresponding twoof the links 21 to each other, and an end effector 23 that is providedat the tip of the arm and attracts (e.g., sucks) and holds a workpiece.

Each of the joint parts 22 is provided with a rotation sensor, such asan encoder, that detects the rotation of that joint part 22, anactuator, such as a servomotor, that drives that joint part 22, and aforce sensor that detects an operational force of that joint part 22.The force sensor is, for example, a torque sensor that detects thetorque of the joint part 22. Each of the joint parts 22 is provided witha speed-reducing mechanism or the like.

The end effector 23 attracts (e.g., sucks) a workpiece in a non-contactmanner, for example, by using an attractive force (e.g., a suckingforce) such as a magnetic force or an air pressure, and holds theattracted workpiece. The end effector 23 is configured to, for example,electromagnetically attract a workpiece by generating a magnetic force,and release the electromagnetically-attracted workpiece by stopping thegenerating of the magnetic force.

The end effector 23 may be configured to grasp a workpiece with a fingerpart or the like. Note that since the holding position of a workpiece iseasily changed three-dimensionally when the end effector 23 isconfigured to attract a workpiece by a magnetic force or the like, theeffect provided by this embodiment that the holding position of theworkpiece can be calculated with high accuracy and the workpiece can beplaced with high accuracy based on the calculated holding position isincreased, which effect will be described later.

The control apparatus 3 performs various arithmetic processes andcontrol processes for the robot arm 2. For example, the controlapparatus 3 uses a processor 3 a such as a CPU (Central Processing Unit)or a GPU (Graphics Processing Unit), an internal memory 3 b such as aRAM (Random Access Memory) and a ROM (Read Only Memory), a storagedevice 3 c such as an HDD (Hard Disk Drive) or an SSD (Solid StateDrive), an input/output I/F (InterFace) 3d for connecting a peripheraldevice such as a display, and a communication I/F 3e for communicatingwith an apparatus located outside the control apparatus.

The first vision sensor 4 is a specific example of first informationacquisition means. The first vision sensor 4 acquires three-dimensionalinformation of a workpiece held by the end effector 23 of the robot arm2. The three-dimensional information of each workpiece includesinformation of the shape of the workpiece, the position (such asthree-dimensional coordinates) thereof, the posture thereof, and thelike. The first vision sensor 4 is composed of a camera, a laser sensor,or the like.

The first vision sensor 4 is provided between the robot arm 2 and theplacement area at substantially the same height position as that of theplacement area. Thus, the first vision sensor 4 can acquirethree-dimensional information of the workpiece, as viewed from the lowerside of the workpiece, held by the end effector 23. Therefore, since asufficient number of pieces of three-dimensional information of theworkpiece can be acquired by only one first vision sensor 4, the numberof sensors can be minimized. This leads to a reduction in apparatuscosts.

Note that a pair of the first vision sensors 4 may be respectivelyprovided on both sides of the end effector 23 so that they sandwich theend effector 23. By the above structure, more accurate three-dimensionalinformation of the workpiece can be acquired. The first vision sensor 4outputs the acquired three-dimensional information of the workpiece tothe control apparatus 3.

The second vision sensor 5 is a specific example of second informationacquisition means. The second vision sensor 5 acquires three-dimensionalinformation of the placement area where the workpiece is to be placed.The three-dimensional information of the placement area includesinformation of the shape and the position (such as three-dimensionalcoordinates) of the placement area, the shape and the position of theworkpiece placed in the placement area, and the like.

The second vision sensor 5 is disposed, for example, in the end effector23 or the link 21 (e.g., one of the links 21) of the robot arm 2. Thesecond vision sensor 5 is composed of a camera, a laser sensor, or thelike. The second vision sensor 5 outputs the acquired three-dimensionalinformation of the placement area to the control apparatus 3.

Conventionally, when a workpiece is held, the holding position of theworkpiece may change three-dimensionally. In this case, for example,there is a possibility that the holding position of the workpiece cannotbe calculated with high accuracy and thus the workpiece cannot be placedin a placement position with high accuracy.

To cope with the above problem, the workpiece holding apparatus 1according to this embodiment acquires by the first vision sensor 4three-dimensional information of the workpiece held by the robot arm 2,calculates a lowest center point of the workpiece as positioninformation of the workpiece based on the acquired three-dimensionalinformation of the workpiece, and calculates a placement position wherethe workpiece is to be placed based on the calculated positioninformation of the workpiece.

By doing the above, it is possible to acquire by the first vision sensor4 three-dimensional information of the workpiece held by the robot arm2, calculate a lowest center point of the workpiece as positioninformation of the workpiece with high accuracy based on the acquiredthree-dimensional information of the workpiece, and place the workpiecein the placement position with high accuracy based on the positioninformation of the workpiece. That is, it is possible to calculate aholding position of a workpiece with high accuracy and place theworkpiece in the placement position with high accuracy based on theholding position.

FIG. 2 is a block diagram showing a schematic system configuration of acontrol apparatus according to this embodiment. The control apparatus 3according to this embodiment includes a position calculation unit 31 anda robot control unit 32.

The position calculation unit 31 is a specific example of positioncalculation means. The position calculation unit 31 calculates positioninformation of a workpiece based on three-dimensional information of theworkpiece acquired by the first vision sensor 4. The positioncalculation unit 31 may calculate a position of the lowest center pointof the workpiece as position information of the workpiece based on thethree-dimensional information of the workpiece acquired by the firstvision sensor 4.

For example, as shown in FIG. 3 , the position of the lowest centerpoint of the workpiece is a position where the center point (the centerof the figure) of the workpiece in a bottom view in which the workpieceis viewed from a lower side thereof is shifted to the height of thelowest point of the workpiece in a side view in which the workpiece isviewed from the side thereof. By calculating the position of the lowestcenter point of the workpiece as the position information of theworkpiece, the workpiece can be grasped at the lowest and center pointof the workpiece where the workpiece is stabilized when it is placed inthe placement position.

For example, the position calculation unit 31 acquires athree-dimensional point cloud of the workpiece from an image of theworkpiece acquired by the first vision sensor 4. The positioncalculation unit 31 calculates the lowest point of the workpiece and thecenter point of the workpiece based on the three-dimensional point cloudof the workpiece. The position calculation unit 31 calculates a pointobtained by shifting the calculated center point of the workpiece to theheight of the lowest point of the workpiece as the position of thelowest center point of the workpiece.

The robot control unit 32 is a specific example of control means. Therobot control unit 32 calculates a placement position of the workpieceheld by the robot arm 2 based on the position information of theworkpiece calculated by the position calculation unit 31. The robotcontrol unit 32 moves the workpiece to the placement position and placesit there by controlling the robot arm 2.

As shown in FIG. 1 , a plurality of spiky projections may be provided inthe placement area of the workpiece. By the above structure, theworkpiece can be reliably prevented from falling over by placing theworkpiece between adjacent projections. The projections are provided,for example, at equal intervals in the XY direction. Each projection isformed so as to have a spiky shape in which the tip thereof is sharp.However, for example, the tip of each projection may be rounded, or eachprojection may have any shape by which the workpiece can be held betweenadjacent projections and thus prevented from falling over.

The robot control unit 32 calculates, for example, a center betweenadjacent projections and a height position of the tip of each of theprojections as the placement position based on the three-dimensionalinformation of the workpiece acquired by the second vision sensor 5.

The first vision sensor 4 acquires information about the heightdirection (Z direction) of the workpiece as three-dimensionalinformation of the workpiece. As shown in FIG. 4 , the information aboutthe height direction of the workpiece is, for example, a length of theworkpiece held by the end effector 23 in the height direction.

The robot control unit 32 changes the placement position of theworkpiece in the placement area based on the information about theheight direction of the workpiece acquired by the first vision sensor 4.In general, when the workpiece is placed in the placement area, thelonger the length of the workpiece in the height direction is, the moreeasily the workpiece falls over. As described above, the ease with whichthe workpiece falls over when the workpiece is placed varies accordingto the length of the workpiece in the height direction. Therefore, theplacement position can be changed to a position where the workpiece isless likely to fall over in accordance with the information about theheight direction of the workpiece, and thus the workpiece can be morereliably prevented from falling over.

As shown in FIG. 1 , a plurality of spiky projections having differentheights may be provided in the placement area. For example, projectionsthat are short in the height direction and projections that are long inthe height direction are provided in the placement area. The secondvision sensor 5 acquires information of the positions of the long andthe short projections in the placement area and the like asthree-dimensional information of the placement area, and outputs it tothe robot control unit 32.

The robot control unit 32 may determine whether or not the length of theworkpiece in the height direction is equal to or greater than apredetermined value t 1 based on the information about the heightdirection of the workpiece acquired by the first vision sensor 4. Asshown in FIG. 4(a), when the robot control unit 32 determines that thelength of the workpiece in the height direction is equal to or greaterthan the predetermined value t 1, the robot control unit 32 sets aposition between the projections that are long in the height directionas the placement position of the workpiece. By doing so, the workpiecethat is long in the height direction is sandwiched between theprojections that are long in the height direction and thus the workpiecethat is long in the height direction is prevented from falling over.

On the other hand, as shown in FIG. 4(b), when the robot control unit 32determines that the length of the workpiece in the height direction isless than the predetermined value t 1, the robot control unit 32 sets aposition between the projections that are short in the height directionas the placement position of the workpiece. By doing so, the workpiecethat is short in the height direction is sandwiched between theprojections that are short in the height direction and thus theworkpiece that is short in the height direction is prevented fromfalling over. The predetermined value t 1 may be set in the robotcontrol unit 32. The predetermined value t 1 is set, for example, basedon the length of the projection in the height direction.

The length of the projection in the height direction may be set inaccordance with the length of the workpiece in the height direction, theworkpiece being placed on the placement area. By doing so, it ispossible to more reliably prevent the workpiece placed between theprojections from falling over. For example, when three workpieces havingdifferent lengths in the height direction are placed in the placementarea, the lengths of three projections in the height direction may beset in accordance with the lengths of the three workpieces in the heightdirection. As described above, any number of different lengths of theprojections in the height direction may be used, and any number ofdifferent lengths of the workpieces in the height direction, whichworkpieces are placed in the placement area, may be used.

The robot control unit 32 may change the placement position in theplacement area based on information about the horizontal direction (Xdirection) of the workpiece acquired by the first vision sensor 4. Forexample, when the robot control unit 32 determines that the length ofthe workpiece in the horizontal direction is longer than a predeterminedvalue t 2, the robot control unit 32 sets a position where no projectionis provided as the placement position of the workpiece. As describedabove, when the workpiece is long in the horizontal direction and thusis less likely to fall over, it is possible to set a position where noprojection is provided as the placement position of the workpiece andtherefore increase the number of possible placement positions of theworkpiece.

The robot control unit 32 may change the placement position of theworkpiece in the placement area based on the information about theheight direction and the horizontal direction of the workpiece acquiredby the first vision sensor 4. For example, when the robot control unit32 determines that the length of the workpiece in the height directionis equal to or greater than the predetermined value t 1 and the lengthof the same in the horizontal direction is equal to or greater than thepredetermined value t 2, the robot control unit 32 sets a positionbetween the projections that are short in the height direction as theplacement position of the workpiece. As described above, when theworkpiece is long in the horizontal direction even though it is alsolong in the height direction, this workpiece is less likely to fallover. Therefore, the workpiece that is long in the height direction maybe disposed so that it is sandwiched between the projections that areshort in the height direction.

Further, the robot control unit 32 sets a position where the workpiecedoes not interfere with another workpiece that is already placed in theplacement area as the placement position of the workpiece based on thethree-dimensional information of the placement area acquired by thesecond vision sensor 5.

As described above, after the workpiece is placed in the placement area,the second vision sensor 5 acquires three-dimensional information of theplacement area, and outputs the acquired three-dimensional informationof the placement area to the robot control unit 32.

For example, as shown in FIG. 5 , the robot control unit 32 acquires athree-dimensional point cloud of workpieces A and B that are alreadyplaced in the placement area based on the three-dimensional informationof the placement area from the second vision sensor 5, and calculates anouter region (dotted line) of each of the workpieces A and B based onthe acquired three-dimensional point cloud.

When a new workpiece is placed in the placement area, the robot controlunit 32 sets the placement position of the workpiece so as to avoid theouter regions of the workpieces A and B in the placement area. By doingso, it is possible to set a position where the workpiece does notinterfere with other workpieces A and B that are already placed in theplacement area as the placement position of the workpiece.

Next, a method for holding a workpiece according to this embodiment willbe described. FIG. 6 is a flowchart showing a flow of processesperformed by the method for holding a workpiece according to thisembodiment. Note that the processes shown in FIG. 6 may be repeated atintervals of a predetermined time.

The first vision sensor 4 acquires three-dimensional information of aworkpiece held by the end effector 23 of the robot arm 2, and outputsthe acquired three-dimensional information of the workpiece to theposition calculation unit 31 of the control apparatus 3 (Step S101).

The position calculation unit 31 calculates the lowest center point ofthe workpiece as position information of the workpiece based on thethree-dimensional information of the workpiece acquired by the firstvision sensor 4, and outputs the calculated position information of theworkpiece to the robot control unit 32 (Step S102).

The second vision sensor 5 acquires three-dimensional information of aplacement area of the workpiece, and outputs the acquiredthree-dimensional information of the placement area to the robot controlunit 32 (Step S103).

The robot control unit 32 calculates a placement position where theworkpiece is to be placed based on the position information of theworkpiece calculated by the position calculation unit 31 and thethree-dimensional information of the placement area acquired by thesecond vision sensor 5 (Step S104).

The robot control unit 32 controls, based on the calculated placementposition, the robot arm 2 so as to move the workpiece to the placementposition and place it there (Step S105).

As described above, the workpiece holding apparatus 1 according to thisembodiment acquires by the first vision sensor 4 three-dimensionalinformation of the workpiece held by the robot arm 2, calculates alowest center point of the workpiece as position information of theworkpiece based on the acquired three-dimensional information of theworkpiece, and calculates a placement position where the workpiece is tobe placed based on the calculated position information of the workpiece.

By doing the above, it is possible to calculate a lowest center point ofthe workpiece as position information of the workpiece with highaccuracy based on the three-dimensional information of the workpieceacquired by the first vision sensor 4, and place the workpiece with highaccuracy based on the position information of the workpiece. That is, itis possible to calculate a holding position of a workpiece with highaccuracy and place the workpiece on a position where the workpiece is tobe placed with high accuracy based on the holding position.

Several novel embodiments according to the present disclosure have beendescribed above. However, these embodiments are merely presented asexamples and are not intended to limit the scope of the disclosure.These novel embodiments can be implemented in various forms. Further,their components/structures may be omitted, replaced, or modifiedwithout departing from the scope and the spirit of the disclosure. Theseembodiments and modifications thereof are included in the scope and thespirit of the disclosure and also included in the disclosure specifiedin the claims and the scope equivalent thereto.

The present disclosure may also be implemented, for example, by having aprocessor execute a computer program and thereby perform the processesshown in FIG. 6 .

The program includes instructions (or software codes) that, when loadedinto a computer, cause the computer to perform one or more of thefunctions described in the embodiments. The program may be stored in anon-transitory computer readable medium or a tangible storage medium. Byway of example, and not a limitation, non-transitory computer readablemedia or tangible storage media can include a random-access memory(RAM), a read-only memory (ROM), a flash memory, a solid-state drive(SSD) or other types of memory technologies, a CD-ROM, a digitalversatile disc (DVD), a Blu-ray disc or other types of optical discstorage, and magnetic cassettes, magnetic tape, magnetic disk storage orother types of magnetic storage devices.

The program may be provided to a computer using any type of transitorycomputer readable media. Examples of transitory computer readable mediainclude electric signals, optical signals, and electromagnetic waves.Transitory computer readable media can provide the program to a computerthrough a wired communication line (e.g., electric wires, and opticalfibers) or a wireless communication line.

Each of the units constituting the control apparatus 3 according to anyof the above-described embodiments is, in addition to being able to beimplemented by the program, able to be partially or entirely implementedby dedicated hardware such as an ASIC (Application Specific IntegratedCircuit) or an FPGA (Field-Programmable Gate Array).

From the disclosure thus described, it will be obvious that theembodiments of the disclosure may be varied in many ways. Suchvariations are not to be regarded as a departure from the spirit andscope of the disclosure, and all such modifications as would be obviousto one skilled in the art are intended for inclusion within the scope ofthe following claims.

What is claimed is:
 1. A workpiece holding apparatus comprising: holdingmeans for holding a workpiece; first information acquisition means foracquiring three-dimensional information of the workpiece held by theholding means; position calculation means for calculating a lowestcenter point of the workpiece as position information of the workpiecebased on the three-dimensional information of the workpiece acquired bythe first information acquisition means; and control means forcalculating a placement position where the workpiece is to be placedbased on the position information of the workpiece calculated by theposition calculation means and controlling, based on the placementposition, the holding means so as to move the workpiece to the placementposition and place the workpiece in the placement position.
 2. Theworkpiece holding apparatus according to claim 1, further comprisingsecond information acquisition means for acquiring three-dimensionalinformation of a placement area where the workpiece is to be placed. 3.The workpiece holding apparatus according to claim 1, wherein the firstinformation acquisition means acquires information about a heightdirection of the workpiece, and the control means changes the placementposition based on the information about the height direction of theworkpiece acquired by the first information acquisition means.
 4. Theworkpiece holding apparatus according to claim 3, wherein a plurality ofspiky projections having different heights are provided in the placementarea where the workpiece is to be placed, the control means sets aposition between the adjacent projections as the placement position ofthe workpiece, and the control means changes the placement position ofthe workpiece in the placement area based on the information about theheight direction of the workpiece acquired by the first informationacquisition means.
 5. The workpiece holding apparatus according to claim4, wherein a length of the projection in a height direction is set inaccordance with a length of the workpiece in the height direction, theworkpiece being placed in the placement area.
 6. A workpiece holdingmethod comprising: acquiring three-dimensional information of aworkpiece held by holding means; calculating a lowest center point ofthe workpiece as position information of the workpiece based on theacquired three-dimensional information; and calculating a placementposition where the workpiece is to be placed based on the calculatedposition information of the workpiece and controlling, based on theplacement position, the holding means so as to move the workpiece to theplacement position and place the workpiece in the placement position. 7.A non-transitory computer readable medium storing a program for causinga computer to perform: a process of acquiring three-dimensionalinformation of a workpiece held by holding means; a process ofcalculating a lowest center point of the workpiece as positioninformation of the workpiece based on the acquired three-dimensionalinformation; and a process of calculating a placement position where theworkpiece is to be placed based on the calculated position informationof the workpiece and controlling, based on the placement position, theholding means so as to move the workpiece to the placement position andplace the workpiece in the placement position.
 8. A workpiece holdingapparatus comprising: a holder configured to hold a workpiece; a firstinformation acquisition unit configured to acquire three-dimensionalinformation of the workpiece held by the holder; a calculator configuredto calculate a lowest center point of the workpiece as positioninformation of the workpiece based on the three-dimensional informationof the workpiece acquired by the first information acquisition unit; anda controller configured to calculate a placement position where theworkpiece is to be placed based on the position information of theworkpiece calculated by the calculator and controlling, based on theplacement position, the holding means so as to move the workpiece to theplacement position and place the workpiece in the placement position.